FileChannel

By / 31 May 2025 / 2 minutes of reading

FileChannel is a class in java.nio.channels that provides a faster and more flexible way to read, write, map, and manipulate files. It works with ByteBuffer and allows for operations like random access, memory-mapped I/O, and file locking.

Unlike InputStream/OutputStream, it supports non-blocking I/O and asynchronous operations.

Opening a FileChannel

Commonly Used Methods

Simple Program: Writing and Reading from a File

import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;

public class FileChannelSimple {
    public static void main(String[] args) throws Exception {
        // Write to file
        RandomAccessFile file = new RandomAccessFile("data.txt", "rw");
        FileChannel channel = file.getChannel();

        String message = "Hello, FileChannel!";
        ByteBuffer buffer = ByteBuffer.allocate(64);
        buffer.put(message.getBytes());
        buffer.flip();
        channel.write(buffer);

        // Read from file
        channel.position(0);
        buffer.clear();
        channel.read(buffer);
        buffer.flip();

        System.out.print("Read: ");
        while (buffer.hasRemaining()) {
            System.out.print((char) buffer.get());
        }

        channel.close();
        file.close();
    }
}

Problem Statement

Mahesh is building a custom logging system for his app.He wants LotusJavaPrince to design a tool that:

  1. Accepts a log message from the user.
  2. Writes the log message with a timestamp to a file log.txt.
  3. Prints the entire log file to the console afterward.

Mahesh insists this be implemented using FileChannel, not BufferedWriter.

import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Scanner;

public class FileChannelLogger {
    public static void main(String[] args) throws Exception {
        Scanner scanner = new Scanner(System.in);

        System.out.print("Enter log message: ");
        String userLog = scanner.nextLine();
        String timeStamp = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(new Date());

        String logEntry = timeStamp + " - " + userLog + "\n";

        // Append the log to file
        RandomAccessFile file = new RandomAccessFile("log.txt", "rw");
        FileChannel channel = file.getChannel();
        channel.position(channel.size()); // move to end for appending

        ByteBuffer buffer = ByteBuffer.wrap(logEntry.getBytes());
        channel.write(buffer);

        // Read and print the entire log file
        channel.position(0);
        buffer = ByteBuffer.allocate((int) channel.size());
        channel.read(buffer);
        buffer.flip();

        System.out.println("\n--- Log File Contents ---");
        while (buffer.hasRemaining()) {
            System.out.print((char) buffer.get());
        }

        channel.close();
        file.close();
        scanner.close();
    }
}

Output

Enter log message: Mahesh logged in

--- Log File Contents ---
2025-05-31 10:22:18 - Mahesh logged inCode language: CSS (css)

The FileChannel class in Java NIO is a powerful and flexible tool for performing file I/O operations with enhanced performance and control. Unlike traditional FileInputStream and FileOutputStream, FileChannel allows both reading and writing, supports random access, and works directly with buffers like ByteBuffer.

It supports key features such as:

  • Memory-mapped files using map() for extremely fast access to large files,
  • Positioned reads/writes for random-access file manipulation,
  • Efficient data transfer between channels via transferTo() and transferFrom(),
  • Locking mechanisms using lock() and tryLock() for safe file access in concurrent environments.

FileChannel is especially beneficial for applications that require high-speed data handling, such as databases, file servers, media processing, and large-scale data manipulation.

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